Air/fuel engine control systems responsive to exhaust gas oxygen sensors positioned both upstream and downstream of a catalytic converter are well known. Various attempts have been made to provide an indication of converter efficiency in response to outputs derived from the upstream and downstream sensors. In one approach a comparison of downstream to upstream sensor amplitudes over a predetermined time provides an indication of converter efficiency. In another known approach, a frequency ratio of downstream to upstream sensor outputs over a predetermined time was used to provide an indication of converter efficiency.
The inventors herein have recognized numerous problems with these prior approaches. For example, variations in the manner in which a vehicle is driven during the predetermined time period in which converter efficiency was tested may result in variations of test results.